In art as in politics, people have begun to see through the charade to the emptiness that lies beneath, says Janet Daley.

By an extraordinary coincidence, two of the most spectacularly successful confidence tricks of the late 20th century are now reaching their endgame within a mile of one another. In Downing Street, the phantasm that was New Labour is disintegrating: having lost Tony Blair – the Great Illusionist, whose personality seemed somehow to make its vacuities and contradictions credible – it is now being exposed as an opportunist scam. Meanwhile, in Trafalgar Square, the cultural fraud which began as a decadent joke by Marcel Duchamp nearly a hundred years ago – that anything can be art providing there are enough conspirators who claim that it is, and enough suckers who believe them – is playing a final, nihilistic hand.

Then again, perhaps it is not a coincidence at all. These two systematic deceits were, when you think of it, quite similar in both their ends and their means. What we are seeing is the simultaneous collapse of politics that isn't politics, and art that isn't art. Both of them had managed, with quite startling effectiveness, to replace the actual substance of their occupation – governing in the case of New Labour, and the creation of works which reflect, and comment on, the human condition in the case of contemporary art – with superbly professional public relations, dazzling, but meaningless rhetoric and brazen self-justification which was sustainable so long as it did not over-reach itself. But with over-confidence came the fall. The palpable failures have been followed by public outrage.

In the case of New Labour, it is directed against the leader left standing exposed in the void. Gordon Brown no longer even pretends to maintain that famous "triangulation" which was supposed somehow to reconcile opposing political values: to erase the conflicts between individual aspiration and the enforcement of equality, between free markets and guaranteed economic security, between the fostering of enterprise and the enforced redistribution of wealth. Unable to maintain the illusion, Mr Brown no longer talks about markets being reconciled with "social justice": he falls back on the real, if discredited, politics of the Left because that at least has a kind of authenticity. What he has not abandoned is the true lesson of New Labour: that governing need not be about what actually happens, but about what you can make people believe has happened. The real legacy of the-politics-that-was-not-politics was that the game was about appearance rather than reality, image rather than substance, media manipulation rather than actual changes in the world.

Over in Trafalgar Square, the growing disgust that is now enveloping Antony Gormley's inflated absurdity of an "artwork" has an air of resigned contempt. The relentless parade of exhibitionists who have been licensed to disport themselves like talentless buskers in one of Britain's most magnificent commemorative spaces, is the least of it. The accompanying panoply of building site structures, wire fencing and safety netting, security and broadcasting paraphernalia has taken over roughly a quarter of the square that was Charles Barry's memorial to a great moment in this nation's history of defying foreign tyranny. And what is this ugly travelling circus celebrating? The logically nonsensical idea that there is no distinction between art and life: a proposition which, were it actually believed, would put an end to the possibility of making art at all. (If everything is art, then nothing is.)

For many who have been simply bemused or faintly exasperated by the defiant narcissistic vacuity of contemporary art, the Gormley project, with its appropriately inane title, One and Other, is a step too far. It is the defiling of a civic space that belongs to the country – which stands for something which many people suddenly feel, perhaps to their surprise, should be treated with dignity and respect. It is not fanciful to suggest that across the minds of many of those onlookers who have hurled sarcastic epithets at the occupants of the fourth plinth, may be running the thought: what sort of country have we become that we allow our national monuments to be treated in this way? Does such an absence of historical pride and dignity possibly bear some connection to the collapse of national self-belief? And a further corollary: is it any wonder that we have so much difficulty persuading the children of immigrants that they should feel proud to be British?

I don't know the answers, but I do know that it is pretty much beyond inconceivable that the Lincoln Memorial in Washington, say, could ever be turned into a platform for performing (or non-performing) nonentities who were encouraged to believe that their mere existence was an art form. (And the US has famously had rather less difficulty persuading its immigrant population to feel pride in their American nationality.)

Now you may feel that this sort of irreverence that Britain manifests towards its history and its institutions is really a sign of healthy cultural confidence. Much the same sort of claim was made in the 1960s when it was fashionable to plaster the Union Jack over everything from carrier bags to coffee mugs: it was salutary that the symbols of patriotism and national glory should not be accorded a kind of quasi-mystical status. Engaging in "flag worship", giving sacramental significance to the icons of nationhood was a sign of insecurity or, worse, a sinister quasi-fascist tendency which could turn viciousin a moment.

Of course, there is something in this. British insouciance can feel very grown-up and sophisticated – and yet, and yet. When does playful impudence slip over into cynicism and alienation? How large a gap is there between not-taking-things-too-seriously and discovering that you no longer know how to take anything seriously? How much can you mock your own history before you become disdainful of it?

So here we are in this state of both disillusion and awakening. What have we discovered? That the easy solutions lead nowhere. In politics, the argument that you didn't have to make hard choices because you could have two opposing things at the same time, ended by achieving nothing.

And in art, the idea that you didn't need to use skill or craft or insight to create anything – you just had to drag a bit of "life" in off the street and display it – was finally exposed in all its emptiness. Now at least we know that what we actually need is real politics which changes the world instead of just rearranging words, and real art which involves an individual imagination observing, reflecting and depicting the life with which we are all trying come to terms. It's a start.